1. Field of the Invention
The present invention relates to thermal lithographic printing plates which are imaged with an infrared laser and processed with an aqueous alkaline developer.
2. Description of Related Art
U.S. Pat. No. 5,340,699 discloses a radiation-sensitive composition especially adapted to prepare a lithographic printing plate that is sensitive to both ultraviolet and infrared radiation and is capable of functioning in either a positive-working or negative-working manner. The disclosed composition is comprised of (1) a resole resin, (2) a novolac resin, (3) a latent Bronsted acid and (4) an infrared absorber. The solubility of the composition in aqueous alkaline developing solution is both reduced in exposed areas and increased in unexposed areas by the steps of imagewise exposure to activating radiation and heating.
U.S. Pat. No. 5,858,626 discloses a lithographic printing plate having a single sensitive layer. The sensitive layer is composed of an infrared imaging composition which contains two essential components, namely an infrared absorbing compound, and a phenolic resin that is either mixed or reacted with an o-diazonaphthoquinone derivative. These compositions are useful in positive-working elements such as lithographic printing plates that can be adapted to direct-to-plate imaging procedures.
WO 97/39894 discloses a lithographic printing plate which contains a lithographic base overcoated with a complex of a developer-insoluble phenolic resin and a compound which forms a thermally frangible complex with the phenolic resin. This complex is less soluble in the developer solution than the uncomplexed phenolic resin. However when the complex is imagewise heated the complex breaks down which allows the non-complexed phenolic resin to dissolve in the developing solution. Thus the solubility differential between the heated areas of the phenolic resin and the unheated areas is increased when the phenolic resin is complexed. Preferably a laser-radiation absorbing material is also present on the lithographic base. Examples of compounds which form a thermally frangible complex with the phenolic resin are disclosed and include quinolinium compounds, benzothiazolium compounds, pyridinium compounds and imidazoline compounds.
WO 99/11458 discloses a lithographic printing plate which contains a support with a hydrophilic surface overcoated with an imaging layer. The imaging layer contains at least one polymer having bonded pendent groups which are hydroxy, carboxylic acid, tert-butyl-oxycarbonyl, sulfonamide, amide, nitrile, urea, or combinations thereof; as well as an infrared absorbing compound. The imaging layer may contain a second polymer which has bonded pendent groups which are 1,2-napthoquinone diazide, hydroxy, carboxylic acid, sulfonamide, hydroxymethyl amide, alkoxymethyl amide, nitrile, maleimide, urea, or combinations thereof. The imaging layer may also contain a visible absorption dye, a solubility inhibiting agent, or both. A method is disclosed for directly imaging the lithographic printing surface using infrared radiation without the requirement of pre- or post- UV-light exposure, or heat treatment. In practice, the imaging layer is imagewise exposed to infrared radiation to produce exposed image areas in the imaged layer which have transient solubility in aqueous alkaline developing solution, so that solubility is gradually lost over a period of time until the imaged areas become as insoluble as non-imaged areas. Within a short time period of the imaging exposure, the imaged layer is developed with an aqueous alkaline developing solution to form the lithographic printing surface. In this method, the infrared radiation preferably is laser radiation which is digitally controlled.
U.S. Pat. No. 5,493,971 discloses lithographic printing constructions which include a grained-metal substrate, a protective layer that can also serve as an adhesion-promoting primer, and an ablatable oleophilic surface layer. In operation, imagewise pulses from an imaging laser interact with the surface layer, causing ablation thereof and, probably, inflicting some damage to the underlying protective layer as well. The imaged plate may then be subjected to a solvent that eliminates the exposed protective layer, but which does no damage either to the surface layer or to the unexposed protective layer lying thereunder.
A heat-sensitive imaging element for making positive working lithographic printing plates is disclosed in European Patent Publication EP 0864420 A1. The imaging element disclosed comprises a lithographic base, a layer comprising a polymeric material which is soluble in an aqueous alkaline solution and an IR-radiation sensitive second layer. Upon image-wise exposure and absorption of IR-radiation in the second (top) layer, the capacity of the aqueous alkaline solution to penetrate and/or solubilize the second layer is changed. Image-wise exposure can be performed with an infrared laser with a short as well as with a long pixel dwell time.
Although advances have been made in the preparation of heat-sensitive elements for the production of lithographic printing plates, there remains a need for such elements having improved sensitivity to infrared laser imaging devices. There is also a need for longer shelf-life with wider development latitude and wider exposure latitude without the production of undesired sludge in the processors.
These needs are met by the present invention which is a positive-working thermal imaging element comprising;
A. a substrate; and
B. a thermally sensitive composite layer structure having an inner surface contiguous to the substrate and an outer surface, the composite layer structure comprising:
(a) a first layer having the inner surface, the first layer comprising a first polymeric material, wherein the first polymeric material is soluble or dispersible in an aqueous solution, and a solubility inhibiting material which reduces the solubility of the first layer in the aqueous solution; and
(b) a second layer having the outer surface, the second layer comprising a second polymeric material, wherein the second layer is insoluble in the aqueous solution, and wherein when the first layer is free of photothermal conversion material, the second layer is free of photothermal conversion material;
wherein, upon heating the composite layer structure, the heated composite layer structure has an increased rate of removal in the aqueous solution.
More particularly, the present invention is a positive-working, lithographic printing plate, precursor comprising;
A. a hydrophilic substrate; and
B. a thermally sensitive composite layer structure having an inner surface contiguous to the hydrophilic substrate and an outer oleophilic surface, the composite layer structure comprising:
(a) a first layer having the inner surface, the first layer comprising a first polymeric material and photothermal conversion material, wherein the first polymeric material is soluble or dispersible in an aqueous solution, and a solubility inhibiting material which reduces the solubility of the first layer in the aqueous solution; and
(b) a second layer having the outer oleophilic surface, the second layer comprising a second polymeric material, wherein the second layer is insoluble in the aqueous solution;
wherein, upon heating the composite layer structure, the heated composite layer structure has an increased rate of removal in the aqueous solution.
An added embodiment of this invention is a method for forming a planographic printing plate comprising the steps, in the order given:
I) providing a lithographic printing plate precursor comprising;
A. a hydrophilic substrate; and
B. a thermally sensitive composite layer structure having an inner surface contiguous to the hydrophilic substrate and an outer oleophilic surface, the composite layer structure comprising:
(a) a first layer having the inner surface, the first layer comprising a first polymeric material, wherein the first polymeric material is soluble or dispersible in an aqueous solution, and a solubility inhibiting material which reduces the solubility of the first layer in the aqueous solution; and
(b) a second layer having the outer oleophilic surface, the second layer comprising a second polymeric material, wherein the second layer is insoluble in the aqueous solution, and wherein when the first layer is free of photothermal conversion material the second layer is free of photothermal conversion material;
II) imagewise exposing the composite layer structure to thermal energy to provide exposed portions and complimentary unexposed portions in the composite layer structure, wherein the exposed portions are selectively removable by the aqueous solution; and
III) applying the aqueous solution to the outer oleophilic surface to remove the exposed portions to produce an imaged lithographic printing plate having uncovered hydrophilic areas of the hydrophilic substrate and complimentary ink receptive areas of the outer oleophilic surface. In an added embodiment of the method of this invention, the imaged lithographic printing plate is uniformly exposed to thermal energy after step III.
In of each of the embodiments of this invention the aqueous solution preferably has a pH of about 6 or greater; the first polymeric material preferably is insoluble in an organic solvent, and the second polymeric material is soluble in the organic solvent; and the first layer preferably contains a photothermal conversion material particularly when the element is imagewise exposed with a radiant source of energy such as an infrared emitting laser. Preferably, the second layer is free of the photothermal conversion material.
This invention relates to an imaging element which can be imaged with thermal energy. More particularly, this invention relates to thermal lithographic printing plates, which can be imaged by thermal energy typically by imagewise exposure with an infrared emitting laser, a thermal printing head, or the like. The lithographic plates described in this invention are made up of a hydrophilic substrate, typically an aluminum or polyester support, and adhered thereto, a thermally sensitive composite layer structure typically composed of two layer coatings. An aqueous developable polymeric mixture containing a solubility inhibiting material, and typically a photothermal conversion material is coated on the hydrophilic substrate to form the first layer. The second layer is composed of one or more non-aqueous soluble polymeric materials which are soluble or dispersible in a solvent which does not dissolve the first layer. As used herein the term xe2x80x9csolubility inhibiting materialxe2x80x9d is intended to include one or more compounds which interact(s) with, or otherwise affects the polymeric compound to reduce the solubility of the first layer in the aqueous solution. In the positive-working thermal imaging element of this invention, the term xe2x80x9cphotothermal conversion materialxe2x80x9d is intended to be one or more thermally sensitive components which absorb incident radiation and convert the radiation to thermal energy. Typically, the photothermal conversion material is an xe2x80x9cinfrared absorbingxe2x80x9d compound. When the first layer contains a photothermal conversion material, i.e., a first material, the second layer may contain the same first material or a different photothermal conversion material, i.e., a second material. As used herein, the term xe2x80x9cthermally sensitivexe2x80x9d is intended to be synonymous with the term xe2x80x9cheat sensitivexe2x80x9d, and the term xe2x80x9cimage area(s)xe2x80x9d is intended to mean the surface area(s) of the imaged plate which is ink-receptive. The plate is exposed in non-image area(s), i.e., areas outside the xe2x80x9cimage areasxe2x80x9d which are not ink-receptive, typically with an infrared laser or a thermal print head. Upon aqueous development of the imaged plate, the exposed portions are developed away thus exposing hydrophilic surfaces of the substrate which are receptive to conventional aqueous fountain solutions. The second layer composed of ink-receptive image areas, protects the underlying aqueous-soluble coating areas from the aqueous developer. In one embodiment of this invention, the second layer may also contain a photothermal conversion material. In this instance, imaging exposure may result in at least partial removal of exposed areas of the second layer from the underlying coating. Any remaining exposed areas of the second layer are removed during development of the imaged plate. In the following description, the invention will be illustrated using infrared radiation, and infrared absorbing material as the photothermal conversion material, but is not intended to be limited thereby. By the use of the solubility inhibiting material in the composite layer structure, solution and development latitude are improved and development can be carried out in a standard processor without production of sludge. In addition, by the use of the composite layer structure of this invention, developability and humidity shelf life are improved relative to single layer, positive-working thermal compositions containing alkali-soluble polymers together with solubility inhibitors.
The plate construction of the present invention includes a composite layer structure supported by a substrate. The composite layer structure contains at least an ink-receptive, aqueous-insoluble second layer overlying an aqueous-soluble infrared absorbing layer which is adhered to the surface of the substrate. The composite structure may additionally contain intermediate layers such as substrate subbing layers to enhance hydrophilicity or adhesion to the composite structure, or an adhesion promoting interlayer between the second layer and the infrared absorbing layer.
Hydrophilic substrates which may be used in the planographic plate of this invention may be any sheet material conventionally used to prepare lithographic printing plates such as metal sheet materials or polymeric sheet material. A preferred metal substrate is an aluminum sheet. The surface of the aluminum sheet may be treated with metal finishing techniques known in the art including brushing roughening, electrochemical roughening, chemical roughening, anodizing, and silicate sealing and the like. If the surface is roughened, the average roughness Ra is preferably in the range from 0.1 to 0.8 xcexcm, and more preferably in the range from 0.1 to 0.4 xcexcm. The preferred thickness of the aluminum sheet is in the range from about 0.005 inch to about 0.020 inch. The polymeric sheet material may be comprised of a continuous polymeric film material, a paper sheet, a composite material or the like. Typically, the polymeric sheet material contains a sub-coating on one or both surfaces to modify the surface characteristics to enhance the hydrophilicity of the surface, to improve adhesion to subsequent layers, to improve planarity of paper substrates, and the like. A preferred polymeric substrate comprises polyethylene terephthalate.
The first layer of the composite layer structure is composed of a polymeric material, a solubility inhibiting material and optionally, a first photothermal conversion material such as an infrared absorbing compound, in which the polymeric material is soluble or dispersible in an aqueous solution having a pH of about 6 or greater, i.e., in a slightly acidic, neutral or alkaline aqueous solution. Useful polymeric materials contain acid functionality and may be composed of one or more polymers or resins. Such polymers and resins include carboxy functional acrylics, acrylics which contain phenol groups and/or sulfonamide groups, cellulosic based polymers and copolymers, vinyl acetate/crotonate/vinyl neodecanoate copolymers, styrene maleic anhydride copolymers, polyvinyl acetals, phenolic resins, maleated wood rosin, and combinations thereof. Typically two polymers are used in combination to achieve the desirable solubility in a wholly aqueous solution having a pH of about 6 or greater and typically between about 8 and about 13.5. Particularly useful in this invention are novolak resins, resole resins and novolakiresole resin mixtures.
Useful polymeric materials are alkali-soluble acrylic resins, which are free of carboxylic acid functionality and which contains at least one of phenolic group, sulfonamide group, N-acylsulfonamide or combinations thereof. Useful acrylic resins of this type include, but are not intended to be limited thereby, a terpolymer of ethyl acrylate, methyl methacrylate and the urea adduct of (1-(1-isocyanato-1-methyl)ethyl-3-(1-methyl)ethenyl benzene)/p-aminophenol reaction product (hereinafter AR-1); a terpolymer of acrylonitrile, methacrylamide and the urea adduct of (1-(1-isocyanato-1-methyl)ethyl-3-(1-methyl)ethenyl benzene)/p-aminophenol reaction product (hereinafter AR-2); a copolymer of acrylonitrile and the urethane adduct of 2-hydroxyethyl methacrylate/p-toluene sulfonyl isocyanate reaction product (hereinafter AR-3); a terpolymer of methacrylamide, N-phenylmaleimide and the urea adduct of (1-(1-isocyanato-1-methyl)ethyl-3-(1-methyl)ethenyl benzene)/p-aminophenol reaction product (hereinafter AR-4); a tetrapolymer of acrylonitrile, methacrylamide, N-phenylmaleimide and the urea adduct of (1-(1-isocyanato-1-methyl)ethyl-3-(1-methyl)ethenyl benzene)/2-amino-4-sulfonamidophenol reaction product (hereinafter AR-5); and a terpolymer of acrylonitrile, methacrylamide and the urea adduct of isocyanatoethyl methacrylate/p-aminophenol reaction product (hereinafter AR-6).
A variety of compounds may be used as solubility inhibiting materials to reduce the solubility of the first layer. Such solubility inhibiting materials (also known as xe2x80x9cdissolution inhibitorsxe2x80x9d) may be reversible insolubilizers or they may be compounds which are capable of irreversibly conversion to solvent soluble components.
Reversible insolubilizers typically have polar or ionic functionality that serve as acceptor sites for hydrogen bonding or weak ionic bond formation with groups on the polymeric material such as hydroxy or carboxylic acid groups. A useful class of reversible insolubilizers are nitrogen containing compounds in which at least one nitrogen atom is quaternized, incorporated in a heterocyclic ring, or quaternized and incorporated in a heterocyclic ring. Examples of useful quaternerized nitrogen containing compounds includes triaryl methane dyes such as Crystal Violet (CI base violet 3), Ethyl Violet and Victoria Blue BO, and tetraalkyl ammonium compounds such as Cetrimide (a C14 alkyl trimethylammonium bromide). A preferred reversible insolubilizer is a nitrogen-containing heterocyclic compound such as quinoline and triazols, e.g., 1,2,4-triazol. Another preferred reversible insolubilizer is a quaternized heterocyclic compound. Examples of suitable quaternized heterocyclic compounds are imidazoline compounds such as Monazoline C, Monazoline O, Monazoline CY, Monazoline T all of which are manufactured by Mona Industries; quinolinium compounds such as 1-ethyl-2-mehtylquinolinium iodide and 1-ethyl-4-mehtyl-quinolinium iodide; benzothiazolium compounds such as 3-ethyl-2-methyl benzothiazolium iodide; and pyridinium compounds such as cetyl pyridinium bromide, ethyl viologen dibromide, and fluoropyridinium tetrafluoroborate. The quinolinium or benzothiazolium compounds may be cationic cyanine dyes such as Quinoldine Blue, 3-ethyl-2-[3-(3-ethyl-2(3H)-benzothiazolylidene)-2-methyl-1-propenyl]benzothiazolium iodide or Dye A having the structure: 
A further useful class of reversible insolubilizers are carbonyl containing compounds such as xcex1-naphthoflavone, xcex2-naphthoflavone, 2,3-diphenyl-1-indeneone, flavone, flavanone, xanthone, benzophenone, N-(4-bromobutyl)phthalimide, and phenanthrenequinone. Formulations useful in preparing the first layer of this invention and which contain reversible insolubilizer compounds are described in WO 97/39894 and U.S. Pat. No. 5,858,626, each being directed to single layer lithographic printing plates.
Solubility inhibiting compounds which are useful in formulating the first layer of this invention may be compounds capable of irreversible conversion to solvent soluble components, such as conventional orthoquinone diazide compounds. Formulations useful in preparing the first layer of this invention and which contain irreversible insolubilizer compounds are described in Sheriff et al., U.S. Pat. No. 5,858,626 which is incorporated herein by reference, and which is directed to single layer lithographic printing plates. Typically an o-diazonaphthoquinone compound is used in admixture with a phenolic resin to form a developer insoluble layer. Alternatively the orthoquinone diazide may be bonded directly to the aqueous solution soluble polymeric material, e.g., through an ester linkage. Upon imaging treatment, the treated areas become soluble in the developer. If the imaging treatment is exposure to ultraviolet radiation the o-diazonaphthoquinone is believed to be irreversibly converted to an indenecarboxylic acid which renders treated areas soluble or dispersible in an alkaline developer. Solubility inhibiting compounds of this type which may be used in the first layer of this invention are o-diazo-naphthoquinone derivatives described in the above mentioned U.S. Pat. No. 5,858,626. The disclosed o-diazonaphthoquinone derivatives are used in admixture with a phenolic resin and an infrared absorbing compound in formulations to form a positive-working lithographic plate. Such o-diazonaphthoquinone derivatives typically comprise an o-diazonaphthoquinone moiety or group attached to a ballasting moiety that has a molecular weight of at least 15, but less than 5000. Examples of such o-diazonaphthoquinone derivatives are esters of 2-diazo-1,2-dihydro-1-oxonaphthalene sulfonic acid or carboxylic acid chlorides. Such useful derivatives include, but are not limited to: 2,4-bis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)benzophenone; 2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy-2,2-bis hydroxyphenylpropane monoester; hexahydroxybenzophenone hexaester of 2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonic acid; 2,2xe2x80x2-bis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)biphenyl; 2,2xe2x80x2,4,4xe2x80x2-tetrakis(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)biphenyl; 2,3,4,-tris(2-diazo-1,2-dihydro-1-oxo-5-naphthalenesulfonyloxy)benzophenone; 2,4-bis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)benzophenone; 2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy-,2-bis hydroxyphenylpropane monoester; hexahydroxybenzophenone hexaester of 2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonic acid; 2,2xe2x80x2-bis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)biphenyl; 2,2xe2x80x2,4,4xe2x80x2-tetrakis(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)biphenyl; 2,3,4,-tris(2-diazo-1,2-dihydro-1-oxo-4-naphthalenesulfonyloxy)benzophenone; and the like such as described in U.S. Pat. No. 5,143,816. In this embodiment the dry weight ratio of phenolic resin to o-diazonaphthoquinone derivative typically is at least 0.5:1, and a weight ratio from about 2:1 to about 6:1 is preferred.
In the alternative embodiment of this invention, the orthoquinone diazide which is a reaction product of the aqueous solution soluble polymeric material (as described above) and an o-diazonaphthoquinone reactive derivative, is used in preparing the first layer. Such a derivative has a functional group (such as chloride or reactive imide group) that can react with a suitable reactive group (for example, a hydroxy group) of the polymeric material (such as a phenolic resin) and thereby become part of the polymeric material, rendering the material sensitive to light. The reactive group can be in the 4- or 5-position of the o-diazonaphthoquinone molecule. Representative reactive compounds include sulfonic and carboxylic acid, ester or amide derivatives of the o-diazonaphthoquinone moiety. Preferred compounds are the sulfonyl chloride or esters, and the sulfonyl chlorides are most preferred. Such reactions with the phenolic resins are described in GB 1,546,633, U.S. Pat. No. 4,308,368 and U.S. Pat. No. 5,145,763. Also useful in the preparation of the first layer are the o-diazonaphthoquinone derivatives of phenolic resins as described in the single layer systems of WO 99/11458 including a condensation polymer of pyrogallol and acetone in which 1,2-naphthoquinone diazide groups are bonded to the phenolic resin through a sulfonyl ester linkage.
In a preferred embodiment of this invention, the first layer contains a first photothermal conversion material such as an infrared absorber. An infrared absorber may be selected from either a dye or pigment. A primary factor in selecting the infrared absorber is its extinction coefficient which measures the efficiency of the dye or pigment in absorbing infrared radiation in accordance with Beer""s Law. The extinction coefficient must have a sufficient value in the wavelength region of infrared radiation exposure usually from 780 nm to 1300 nm. Examples of infrared absorbing dyes useful in the present invention include, Cyasorb IR 99 and Cyasorb IR 165 (both available from Glendale Protective Technology), Epolite IV-62B and Epolite III-178 (both available from the Epoline Corporation), PINA-780 (available from the Allied Signal Corporation), Spectra IR 830A and Spectra IR 840A (both available from Spectra Colors Corporation), ADS 830A and ADS 1060A (ADS Corp) and EC 2117 (FEW Wolfen). Examples of infrared absorbing pigments are Projet 900, Projet 860 and Projet 830 (all available from the Zeneca Corporation). Carbon black pigments may also be used. Carbon black pigments are particularly advantageous due to their wide absorption bands since such carbon black-based plates can be used with multiple infrared imaging devices having a wide range of peak emission wavelengths.
In one embodiment of this invention the solubility inhibiting material is the photothermal conversion material. Illustrative of such a material having a dual function is Dye B having the formula: 
which is used in the single layer formulations described in Haley et al., U.S. Pat. No. 5,340,699, which is incorporated herein by reference.
The second layer of the composite layer structure, i.e. the top layer, is insoluble in the aqueous solution having a pH of about 6 or greater, and contains as an essential ingredient a polymeric material which is ink-receptive and soluble or dispersible in a solvent such as an organic solvent or an aqueous solvent dispersion. Preferably, the polymeric material itself is insoluble in the aqueous solution having a pH of about 6 or greater. Useful polymers of this type include acrylic polymers and copolymers; polystyrene; styrene-acrylic copolymers; polyesters, polyamides; polyureas; polyurethanes; nitrocellulosics; epoxy resins; and combinations thereof. Preferred are polymethylmethacrylate, nitrocellulose and polystyrene. When the first layer contains a photothermal conversion material, the second layer may also contain a photothermal conversion material, which typically is the same infrared absorbing dye which is used as the photothermal conversion material in the first infrared absorbing layer. The second layer may also contain a dye or pigment, such as a printout dye added to distinguish the exposed areas from the unexposed areas during processing; or a contrast dye to distinguish image areas in the finished imaged plate. The second layer may also contain polymeric particles which are incompatible with the second polymeric material. As used herein the term xe2x80x9cincompatiblexe2x80x9d is intended to mean that the polymeric particles are retained as a separate phase within the second polymeric material. Typically, the polymeric particles have an average diameter between about 0.5 xcexcm and about 10 xcexcm. Preferred polymeric particles of this type are poly tetrafluoroethylene particles. The presence of such polymeric particles improves scratch resistance of the composite layer and surprisingly enhances exposure latitude for processing the plate. Typically, the second layer is substantially free of ionic groups.
The composite layer structure may be applied to the substrate by sequentially applying the first layer and then the second layer using conventional coating or lamination methods. Alternatively, both layers may be applied at the same time using multiple layer coating methods such as with slot type coaters; or from a single solution which undergoes self-stratification into top and bottom layers upon drying. However it is important to avoid substantial intermixing the two layers which tends to reduce the sensitivity. Regardless of the method of application, the first layer of the applied composite has an inner surface which is contiguous to the substrate, and the second layer of the applied composite has an outer surface.
The first layer may be applied to the hydrophilic substrate by any conventional method. Typically the ingredients are dissolved or dispersed in a suitable coating solvent, and the resulting solvent mixture is coated by known methods such as by whirl coating, bar coating, gravure coating, roller coating, and the like. Suitable coating solvents include alkoxyalkanols such as 2-methoxyethanol; ketones such as methyl ethyl ketone; esters such as ethyl acetate or butyl acetate; and mixtures thereof.
The second or top layer may be applied to the surface of the thermal conversion layer by any conventional method such as those described above. Typically the ingredients are dissolved or dispersed in a suitable organic coating solvent which is not a solvent for the thermal conversion layer. Suitable coating in solvents for coating the second layer include aromatic solvents such as toluene and mixtures of aromatic solvents with alkanols such as a 90:10 weight ratio of toluene and butanol.
Alternatively, the first layer, the second layer or both layers may be applied by conventional extrusion coating methods from a melt mixture of layer components. Typically, such a melt mixture contains no volatile organic solvents.
The thermal digital lithographic printing plate precursor is imaged by the method comprising the following steps. First a lithographic printing plate precursor is provided which comprises a hydrophilic substrate and adhered thereto, a composite layer structure having an inner surface contiguous to the hydrophilic substrate and an outer oleophilic, ink-receptive surface. The composite layer structure comprises a first layer which forms the inner surface of the composite layer structure and a second layer which forms the outer surface of the composite layer structure. The first layer comprises a first polymeric material; a solubility inhibiting material and a photothermal conversion material, as previously described, in which the first polymeric material is soluble or dispersible in an aqueous solution having a pH of about 6 or greater, and the solubility inhibiting material reduces the solubility of the first layer. The second layer consists essentially of a second polymeric material, as previously described, which is soluble in the organic solvent, wherein the second layer is insoluble in the aqueous solution. Next the composite layer structure is imagewise exposed to thermal energy to provide exposed portions, or areas, and complimentary unexposed portions, or areas, in the composite layer structure. The exposed portions surprisingly are selectively removable by the aqueous solution. Finally, the aqueous solution is then applied to the outer oleophilic surface to remove the exposed portions of the composite layer structure to produce an imaged lithographic printing plate. The resulting imaged lithographic printing plate has uncovered hydrophilic areas of the hydrophilic substrate and complimentary ink receptive areas of the outer oleophilic surface. While not being bound by any particular theory, selective removability of the exposed portions is believed to result from an increased rate of dissolution or dispersibility of the first layer in the aqueous solution, from enhanced permeability of the second layer to the aqueous solution or to a combination thereof. The printing plates of this invention have a distinct advantage over other lithographic printing plate systems, since the plates of this invention possess useful development latitude without the need for pre-development conditioning such as pre-heating prior to development.
The lithographic plate of this invention and its methods of preparation have already been described above. This plate may be imaged with a laser or an array of lasers emitting infrared radiation in a wavelength region that closely matches the absorption spectrum of the first infrared absorbing layer. Suitable commercially available imaging devices include image setters such as a Creo Trendsetter (available from the CREO Corporation, British Columbia, Canada) and a Gerber Crescent 42T (available from the Gerber Corporation). While infrared lasers are preferred other high intensity lasers emitting in the visible or ultraviolet may also be used to image the lithographic plate of this invention. Alternatively, the lithographic plate of this invention may be imaged using a conventional apparatus containing a thermal printing head or any other means for imagewise conductively heating the composite layer such as with a heated stylus, with a heated stamp, or with a soldering iron as illustrated in the following examples.
When portions of the composite layer structure are exposed to infrared radiation, they become selectively removable by an aqueous developer liquid and are removed thereby. The developer liquid may be any liquid or solution which can both penetrate the exposed areas and dissolve or disperse the exposed areas of the infrared absorbing layer without substantially affecting the complimentary unexposed portions of the composite layer structure. Useful developer liquids are the aqueous solutions having a pH of about 6 or above as previously described. Preferred developer solutions are those that have a pH between about 8 and about 13.5. Useful developers include commercially available developers such as PC3000, PC955, PC956, PC4005, PC9000, and Goldstar DC aqueous alkaline developers each available from Kodak Polychrome Graphics, LLC. Typically the developer liquid is applied to the imaged plate by rubbing or wiping the second layer with an applicator containing the developer liquid. Alternatively, the imaged plate may be brushed with the developer liquid or the developer liquid may be applied to the plate by spraying the second layer with sufficient force to remove the exposed areas. Alternatively, the imaged plate can be soaked in the developer liquid, followed by rubbing or brushing the plate with water. By such methods a developed printing plate is produced which has uncovered areas which are hydrophilic and complimentary areas of the composite layer, not exposed to infrared radiation, which are ink receptive.
Although lithographic printing plates having high press life with good ink receptivity are produced at high imaging speeds by the method of this invention, press life surprisingly is further enhanced by uniformly exposing the imaged lithographic printing plate to thermal energy after it has been developed in step III. Such a uniform thermal exposure may be carried out by any conventional heating technique, such as baking, contact with a heated platen, exposure to infrared radiation, and the like. In a preferred mode for post development thermal exposure, the developed imaged lithographic printing plate is passed through a baking oven at 240xc2x0 C. for 3 minutes after treatment with a baking gum.